Magnetic lens



Aug. 11, 1942-. ER 2,292,877

MAGNETIG'LENS Filed Oct. 1 6, 1940 2 Sheets-Sheet 1 3 2'5 i l i HG" /Z7HG. 4'

Z1 Z1 2a I Z1 21 A .37

35 3nnentor 3g 9 Mn 21 19 James Hillier Aug. 11, 1942. J H|| |ER2,292,877

. MAGNETIC LENS Filed Oct. 1e, 194o 2 Sheets-Sheet 2 77 2 0 ZhmcntotJames Hillier Patented Aug. 11, 1942 James Hillier,.Collingswood, N. J

assignor to I .Radio Corporation of America, a corporation of DelawareApplication center 16, 1940, Serial No. 361,348

Claims. (01. 250-495) This invention relates to magnetic lenses of thetype used in electron microscopes and theclike and especially to themethod or process of making the pole pieces for such lenses.

It has been shown that a magnetic field may be used to focus electronrays in a manner analogous to the focusing of light rays by an opticallens. By a suitable arrangement of lenses, a magnified image of anobject may be formed by' electron rays passing through the object.Magnifications of the order of twenty or more thousand times have beenused. Magnifications of that order require extremely accurate lenses ifobjectionable aberrations are to be avoided.

The pole pieces of magnetic lenses should possess axial symmetry and theaxes should be collinear. These conditions may be best met by turningall the active surfaces of the'pole pieces 'in the manner hereinafterdescribed.

It is therefore an object of this invention to provide a method ofmanufacturing magnetic pole pieces for electron lenses and the like.Another object is to provide a method for machining magnetic pole piecesfrom one piece of material in a series of steps which assure accuratesymmetrical surfaces and a collinear axis; An additional object is toprovide a method for machining the pole pieces of a magnetic lens andfor maintaining a collinear relation by means of a non-magneticmaterial. A still further object is to provide means for retaining thealignment of a pair of pole pieces of a magnetic lens to form a unitarystructure.

The invention will be described by referring to the accompanyingdrawings, in which Figure l is a sectional view of a preferredembodiment of a magnetic lens; Figures 2, 3, 4 and 5 are sectional viewsillustrating the method of machining the pole pieces; Figure 6 is asectional view of a modified magnetic lens; and Figures '7, 8, 9 and 10are sectional views illustrating a modified method of machining the polepieces.

Referring to Fig. 1, a magnetic lens is formed by surrounding a solenoidwinding I with a magnetic shell 3. The shell includes a base plate 5which has an inwardly extending coaxial tubular portion 1, a cylindricalouter shell 9 and an annular top plate II. The parts of the shell arepreferably held together by drive fits; or mechanical fastening means,such as screws or threaded parts, may be used. The lens is completed bypole pieces l3, which include a pair of symmetrical members, maintainedin spaced relation by a non-magnetic tubular spacer l5 to form a unitarystructure. The shell, which is made of magnetic material, and the polepieces are held together by a drive fit or any suitable means.

The pole pieces are machined from a single blank which is secured to theclutch jaws H) of a lathe, automatic screw machine or the like. Thefirst operation is to turn the blank to form the outer walls 2|,inwardly extending sloping side walls 23, sloping top wall 25 and flattop wall 21. The second operation is to bore a hole 29 along the axis 3|to a depth not less than the final length of the inner cylindrical wallsof the pole piece. The'third step is to shrink and insert within saidhole a non-magnetic retainer or spacer so that it forms a tight fit uponexpansion. The non-magnetic retainer is drilled along its axis to forman opening corresponding to the finished inner cylindrical walls of thepole pieces. The blank after the completion of the third step is shownin Fig. 3. The fourth step is to divide the magnetic portions of thepole pieces by cutting the gap 33 (see Fig. 4) from the sloping sidewalls 23 to th outer portion of the nonmagnetic tubular retainer I5. Thefifth and final step is to cut from the outer wall 2| inwardly to formthe lower fiat bottom wall 25 and the bottom inwardly sloping wall 31.

It should be understood that the invention contemplates a reversal ofparts; i. e., the nonmagnetic retainer may surround the pole piecesinstead of having th pole pieces surround the retainer. If the parts arethus reversed, the lens will appear as shown in Fig. 6 in which asolenoid coil 41 is surrounded by a shell 43 of magnetic material. Theshell includes a base plate 45, an inwardly extending tubular portion41, an outer cylindrical member 49, and an annular top plate 5!. Thepole pieces 53 are tightly fitted between.

the top plate 5| and th tubular portion 41.

The method of machining the pole pieces will be described by referringto Figs. '7, 8, 9 and 10. A blank 51 is fastened to a lathe or the likeby means of chuck jaws 59. The blank is ma:- chined to form the outercylindrical wall 6|, including inwardly sloping portion 63. The inwardlysloping portion 63 is extended by cutting parallel to the flat plate 65to form a gap 69. The end of the blank is cut to form a flat plate 65and an inwardly sloping portion 61. The next step is to fit tightly anon-magnetic tubular retaining member H around the cylindrical Wall 6|.The retainer is preferably shorter than the cylindrical wall so that themagnetic portion of the upper pole piece 53 contacts the annular plate5| to complete the magnetic circuit.

The blank is bored along its axis to form the inner cylindrical surfaces13 of the pole pieces. The next steps are to cut the blank at the lowerportion of the pole pieces to form a bottom wall 15 parallel to theannular top plate 65 and finally to cut the inwardly sloping portion 11.

In both the present method and the first described or preferred methodthe separating of the pole pieces from the blank may be done by clampingthe pole pieces in a collet or other chuck so that the bottom wall andsloping wall portion may be machined from the front instead of the rear.Any slight inaccuracy caused by thus removing the piece from theoriginal position for the cutting off operation will have no observableeffect on the lens because the lens field is essentially located betweenthe inner cylindrical walls and adjoining parallel walls.

Since all of the foregoing operations may be performed without removingthe blank from the machine, it follows that the pole pieces may be madesymmetrical, and collinear. Furthermore, the foregoing steps are simpleturning operations which can be performed accurately and inexpensively.The non-magnetic tubular portion retains the pole pieces in the desiredalignment Without affecting the field and makes the pole pieces aunitary structure.

I claim as my invention:

1. The process of machining pole pieces from a single blank of magneticmaterial and a nonmagnetic retainer which includes machining said blankto form a portion of the walls of said pole pieces, fitting saidnon-magnetic retainer to said machined walls to hold securely togetherthe upper and lower portions of said pole pieces, forming the innercylindrical wall of said pole pieces, and separating the thus machinedpole pieces from the balance of said blank bu cutting the bottom wall ofthe lower portion of said pole piece.

2. The process of machining pole pieces from a single blank of magneticmaterial and a nonmagnetic retainer which includes machining said blankto form some of the cylindrical and end walls of said pole pieces,fitting said non-magnetic retainer to said cylindrical walls to holdsecurely together the upper and lower portions of said pole pieces,forming the inner cylindrical wall of said pole pieces, and separatingthe thus machined pole pieces from the balance of said blank by cuttingthe bottom wall of the lower portion of said pole piece.

3. The method of machining a unitary symmetrical and collinear polepiece structure from a single blank of magnetic material and anonmagnetic retainer which includes the steps of turning said blank toform an outer wall and a top wall, drilling a hole into said blank alongthe axis of said turning from said top wall to a bottom wall to beformed, driving a non-magnetic retainer in said hole, drilling a hole insaid retainer to form the inner cylindrical wall of said pole pieces,forming an upper and a lower pole piece by cutting a gap in said blankfrom said outer wall to the outer portion of said spacer, and separatingthe thus formed pole piece from said blank by cutting through said blankto form a bottom wall for said lower pole piece.

4. The method of machining a unitary symmetrical and collinear polepiece structure from a single blank of magnetic material and anonmagnetic retainer which includes the steps of turning said blank toform an outer wall and a top wall, drilling a hole into said blank alongthe axis of said turning from said top wall to a bottom wall to beformed, driving a non-magnetic retainer in said hole, drilling a hole insaid retainer to form the inner cylindrical wall of said pole pieces,forming an upper and a lower pole piece by cutting a gap in said blankfrom said outer wall to the outer portion of said spacer and parallel tosaid top wall, and separating the thus formed pole piece from said blankby cutting through said blank to form a bottom wall for said lower polepiece, said bottom wall being parallel to said top wall.

5. The method of forming pole pieces from a single blank of magneticmaterial and a nonmagnetic retainer which includes the steps of turningthe outer walls, inwardly sloping top wall and fiat top wall, drilling acoaxial aperture from said sloping top wall through said blank, fittinga non-magnetic retainer within said aperture, drilling a coaxialaperture through said retainer to form the inner cylindrical walls ofsaid pole pieces, cutting a gap from said inwardly sloping walls to theouter surface of said retainer, cutting said blank to form a bottom flatwall and cutting an inwardly sloping wall from said bottom flat wall tosaid retainer to separate the thus machined pole pieces from the balanceof said blank.

JAMES HILLIER.

